1. Field of the Invention
The present invention relates, in general, to a spatial light modulator using a near-field and, more particularly, to a spatial light modulator using a near-field, in which the near-field formed between two media having different refractive indices and being in contact with each other is transformed to reflect and diffract incident light.
2. Description of the Prior Art
Generally, an optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a great amount of data in real time. Studies have been conducted on the design and production of a binary phase only filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.
The spatial light modulator is applied to optical memory, optical display device, printer, optical interconnection, and hologram fields, and studies have been conducted to develop a display device employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The light modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The light modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspended above an upper part of a silicon substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14 are deposited.
The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on the oxide spacer layer 12 by a nitride frame 20.
In order to modulate light having a single wavelength of λo, the modulator is designed so that thicknesses of the ribbon 18 and oxide spacer 12 are each λo/4.
Limited by a vertical distance (d) between a reflective surface 22 of each ribbon 18 and a reflective surface of the substrate 16, a grating amplitude of the modulator 10 is controlled by applying a voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 formed on a lower side of the substrate 16 to act as a second electrode).
In an undeformed state of the light modulator with no voltage application, the grating amplitude is λo/2 while a total round-trip path difference between light beams reflected from the ribbon and substrate is λo. Thus, a phase of reflected light is reinforced.
Accordingly, in the undeformed state, the modulator 10 acts as a plane mirror when it reflects incident light. In FIG. 2, reference numeral 20 denotes the incident light reflected by the modulator 10 in the undeformed state.
When a proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to move downward toward the surface of the substrate 16. At this time, the grating amplitude is changed to λo/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference.
The modulator diffracts incident light 26 using the interference. In FIG. 3, reference numerals 28 and 30 denote light beams diffracted in +/−diffractive modes (D+1, D−1) in the deformed state, respectively.
However, the conventional light modulator is problematic in that the gaps that always exist between ribbons while the ribbons are not moved bring about a loss of light incident on the gaps between the ribbons.
Other problems of the conventional light modulator are that since the gaps must be minimized in order to minimize the light loss caused by the gaps between the ribbons, a structure is complicated, productivity is reduced, and precision is reduced.
Furthermore, the conventional light modulator is problematic in that diffraction always occurs due to the gaps between the ribbons even though the ribbons are not moved, resulting in poor contrast.